WO2012164440A1

WO2012164440A1 - A led-based illumination device with low heat up color shift

Info

Publication number: WO2012164440A1
Application number: PCT/IB2012/052567
Authority: WO
Inventors: Tewe Hiepke Heemstra
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-06-01
Filing date: 2012-05-22
Publication date: 2012-12-06

Abstract

An illumination device (1), comprising: a phosphor-based white light emitting diode (2) and a red light emitting diode (3) configured so that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature; and a positive temperature co-efficient resistor (7) electrically connected in parallel with the red light emitting diode (3), and thermally connected to the white (2) and red (3) light emitting diodes such that the temperature of the positive temperature co-efficient resistor (7) changes with the junction temperatures of the white (2) and red (3) light emitting diodes, wherein the positive temperature co-efficient resistor (7) is configured to reduce the color shift relative to the pre-defined color point during start-up of the illumination device below a predetermined threshold.

Description

A LED-based illumination device with low heat up color shift

FIELD OF THE INVENTION

The present invention relates to an illumination device, comprising a phosphor-based white light emitting diode and a red light emitting diode configured such that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature. The invention also relates to a method of manufacturing such an illumination device.

BACKGROUND OF THE INVENTION

Illumination devices based on light emitting diodes (LEDs) are increasingly used to provide energy efficient illumination. For instance, LED-based retrofit lamps can be used to replace incandescent lamps. To reproduce the colors of various objects faithfully in comparison with an ideal or natural light source a high color rendering index (CRI) is desirable. For LED-based lamps that use a phosphor-based white light emitting diode this may be achieved by including a red light emitting diode. To provide a stable color point, the red flux should be in a substantially fixed ratio to the phosphor pumped white flux. However, as the hot/cold factor (i.e. the reduction in light output for a constant input current due to rising junction temperature) is substantially higher for red light emitting diodes than for phosphor-based white light emitting diodes, the color point of the illumination tends to change as the junction temperature of the light emitting diodes rises during start-up of the illumination device resulting in a color shift. The color shift is particular prominent for retrofit lamps which are very compact and difficult to cool, and thus get really hot. Thus, for retrofit lamps, the heating-up color shift can be over 25 SDCM (Standard Deviation of Colour Matching). Humans are very sensitive to such color variations, and color differences above 5 SDCM are generally perceived as annoying.

From patent document US 2008/0136331 it is known to use a temperature sensing element for sensing an operating temperature of a light-emitting element via a probe, and a driving system operatively coupled to the temperature sensing element and the light- emitting element, wherein the driving system is configured to control the light-emitting element using the sensed operating temperature in order to maintain a desirable light output characteristics.

However, although such a solution can be used to compensate for the shift in color during start-up of an LED-based illumination device, the solution is rather complex and increases the cost of the illumination device. Therefore, there is a need for a simple and cost- efficient solution to reduce the heating-up color shift during start-up of the illumination device.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem, and to provide a simple and cost-efficient solution that alleviates the problem of heating-up color shift during start-up of the illumination device.

According to an aspect of the invention, this and other objects are achieved by an illumination device, comprising:

a phosphor-based white light emitting diode and a red light emitting diode configured such that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature; and

a positive temperature co-efficient resistor electrically connected in parallel with the red light emitting diode, and thermally connected to the white and red light emitting diodes such that the temperature of the positive temperature co-efficient resistor changes with the junction temperatures of the red and white light emitting diodes,

wherein the positive temperature co-efficient resistor is configured to reduce the color shift relative to the pre-defined color point during start-up of the illumination device below a predetermined threshold. The predetermined threshold may preferably be 5 SDCM (Standard Deviation of Colour Matching) to avoid that the color shift is perceived as annoying by a human when the illumination device is used in a lighting application, or more preferably even lower, such as 3 SDCM, or most preferably 1.5 SDCM.

By color shift is here intended the deviation in color point between the predefined color point of the illumination device that is achieved at normal operating

temperature and the color point of the combined light emitted by the white and red light emitting diodes during start-up. By start-up is intended the time from when the illumination device is switched on, and the light emitting diodes have a junction temperature about room temperature, until the light emitting diodes has reached their normal operating temperature, which is typically a junction temperature about 80°C to 110°C.

The present invention is based on the realization that by using a positive temperature co-efficient resistor electrically connected in parallel to the red light emitting diode, and in thermal connection with the junctions of the white and red light emitting diodes, and configuring the positive temperature co-efficient to account for the heat-up behavior of the red and white light emitting diode a substantial reduction in color shift during start-up can be achieved. Thus, in response to rising junction temperatures of the white and red light emitting diodes during start-up of the illumination device, the temperature of the positive temperature co-efficient resistor is increased so that the input current to the red light emitting diode is increased, so as to compensate for the larger efficiency reduction of the red light emitting diode compared to the white light emitting diode, thereby reducing the color shift. An advantage is that the solution removes interactions between the light emitting diodes and the power supply that would be required by alternative solutions, for example the connection of a thermocouple. Thereby, if the PTC is enclosed in the LED package or if the PTC is sold together with the LED or LEDs, the color shift problem can be solved substantially at the LED manufacturer, and does not have to be handled at the system integrator.

A second positive temperature co-efficient resistor may preferably be electrically connected in parallel with the red light emitting diode and the first positive temperature co-efficient resistor. It has been found that by using two positive temperature coefficient resistors it is possible to more accurately account for the heat-up behavior of the red and white light emitting diodes, thereby further reducing the color shift during start-up.

To provide a low color shift during start-up, the resistance of the first positive temperature co-efficient resistor and the resistance of the second positive temperature coefficient resistor may preferably substantially coincide in a temperature range below the switching temperatures, e.g. from room temperature to a temperature about the lower switching point.

The phosphor-based white light emitting diode and the red light emitting diode may preferably be connected in series. An advantage is that the red and white light emitting diodes do not have to be controlled separately, thereby enabling a more cost-efficient solution.

The positive temperature co-efficient resistor(s) may be a layer(s) on which the white and/or red light emitting diode is/are arranged. The light emitting diodes can be mounted to the layer by means of soldering. However, the positive temperature co-efficient resistor may also be a component arranged next to the light emitting diodes, and thermally connected to the light emitting diodes e.g. via a thermal path on a substrate on which the white and red light emitting diodes are arranged. The thermal path may be of copper or other suitable material of high thermal conductivity. Alternatively, or additionally, one or more positive temperature co-efficient resistor may be configured inside the LED component, and be implemented as a block or a layer. It may enable not applying a separate TVS (transient voltage suppressor) within or outside of the LED package, providing a cost advantage.

The illumination device may further comprise at least one additional phosphor-based white light emitting diode connected in series with the first phosphor-based white light emitting diode to form a set of phosphor-based white light emitting diodes, and at least one additional red light emitting diode connected in series with the first red light emitting diode to form a set of red light emitting diodes, wherein the positive temperature coefficient resistor(s) is or are electrically connected in parallel with the set of red light emitting diodes, and wherein the set of phosphor-based white light emitting diodes are electrically connected in series with the set of red light emitting diodes and the positive temperature coefficient resistor(s). An advantage is that the number of components can be minimized to enable a cost-efficient solution. Also, a higher voltage can be used thereby reducing the loss of power. Further, the risk of thermal runaway that would exist with a number of LEDs configured in parallel is eliminated. When a positive temperature co-efficient resistor is implemented as layer, patterning the layer into a number of positive temperature co-efficient resistors may not substantially influence cost, so in that case it may be advantageous to implement it as multiple positive temperature co-efficient resistors.

The phosphor-based white light emitting diode may be an InGaN LED arranged to illuminate phosphor. The phosphor may be applied directly on the die or be a remote phosphor, or a combination. The remote phosphor(s) may have different temperatures than the LED junctions.

The red light emitting diode may be a GaP, AlGaAs, GaAsP, AlGalnP, organic or phosphor pumped LED, or any other LED with relevant change in light output versus temperature.

The illumination device may comprise a further positive temperature coefficient resistor electrically connected in parallel with both the phosphor-based white light emitting diode and the red light emitting diode and thermally connected to the light emitting diodes to vary the input current to the illumination device. An advantage is that a substantially constant total flux of the combined light from the phosphor-based white light emitting diode and the red light emitting diode can be maintained over time. This allows a compensation for the overall variation of flux (otherwise the flux would be slightly higher at startup). An additional advantage of this further positive temperature resistor is that the illumination device may get a 'soft start' instead of instantaneously burning at nominal output, depending on the chosen configuration of the positive temperature co-efficient resistor.

According to an embodiment, the illumination device is a retro-fit lamp. A retro-fit lamp here refers to an LED-based lamp that can be used to replace an incandescent lamp.

The illumination device may advantageously be included in a luminaire.

According to another aspect of the invention, there is provided a method of manufacturing an illumination device, comprising the steps of: providing a phosphor-based white light emitting diode and a red light emitting diode configured so that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature; providing at least one positive temperature co-efficient resistor electrically connected in parallel with the red light emitting diode, and thermally connected to the white and red light emitting diodes so that the temperature of the positive temperature co-efficient resistor(s) changes with the junction temperatures of the red and white light emitting diodes; wherein the positive temperature coefficient resistor(s) is or are configured so that the color shift of the combined light emitted by the white emitting diode and the red light emitting diode is below a predetermined threshold during start-up. This aspect of the invention provides similar advantages as the previous aspect of the invention.

The positive temperature co-efficient resistor(s) may be selected by: providing a model of how light output of the phosphor-based white light emitting diode varies with rising junction temperature; providing a model of how light output of the red light emitting diode varies with rising junction temperature; providing a model(s) of how the resistance of the positive temperature co-efficient resistor(s) varies with temperature; using the models to simulate the color shift of the illumination device during start-up by varying the temperature; and optimizing the parameters of the positive temperature co-efficient resistor(s) to achieved a color shift below the predetermined threshold. The parameters of the positive temperature co-efficient resistor(s) may include the switching temperature and/or the resistance at room temperature. The method may further comprise the steps of providing a set (or batch) of available light emitting diodes and positive temperature co-efficient resistors; measuring properties of individual light emitting diodes and positive temperature co-efficient resistors in said set of available light emitting diodes and positive temperature co-efficient resistors; performing an optimization based on the measured properties to find combinations of the set of available light emitting diodes and positive temperature co-efficient resistors that maximize the fraction of the available light emitting diodes and positive temperature coefficient resistors usable to manufacture illumination devices with a color shift below the predetermined threshold; and assembling the light emitting diodes and positive temperature co-efficient resistors according to the resulting combinations to manufacture illumination devices with a color shift below the predetermined threshold. An advantage is that the available components can be matched into optimal combinations, whereby a higher accuracy can be achieved than for a conventional binning procedure.

Preferably, the method may further comprise the steps of stopping the assembling process when a mishap occurs that renders a light emitting diode or positive temperature co-efficient resistor unusable; re-starting the optimization based on the, at that time, available light emitting diodes and positive temperature co-efficient resistors; and continuing assembling of the light emitting diodes and positive temperature co-efficient resistors according to the resulting combinations of the new optimization. An advantage is that as only the defect component is discarded, as the other components that were to be combined with the defect component can be combined with other components during the re- optimization.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 is a schematic perspective view of an illumination device according to an embodiment of the invention.

Fig. 2 is a schematic circuit diagram of the illumination device in Fig. 1.

Fig. 3 illustrates an example of how the resistance of the PTC resistors varies as a function of temperature. Fig. 4 is a flow chart illustrating an embodiment of a method of manufacturing the illumination device in Fig. 1.

Fig. 5 is a flow chart illustrating an embodiment of a method of selecting the PTC resistors so that the color shift is below a predetermined threshold during start-up of the illumination device.

Fig. 6 is a flow chart illustrating a method that finds combinations of components, from a batch of components, that maximize the fraction of the components in the batch usable to manufacture illumination devices with a color shift below the

predetermined threshold.

Figs. 7a-c illustrates the color drift during start-up in CIE color space for an illumination device with two PTC resistors, without any PTC resistor, and with one PTC resistor, respectively.

Fig. 8 is a schematic circuit diagram of an embodiment of an illumination device with a plurality of white light emitting diodes and a plurality of red light emitting diodes.

DETAILED DESCRIPTION

With reference to Figs. 1-7 an illumination device according to a currently preferred embodiment of the present invention will now be described. This embodiment relates to an LED-based lamp intended as a retrofit for incandescent lamps. However, the illumination device may also be used for other types of LED-based illumination devices, such as luminaires, downlighters, spots, stage lighting, retrofit kits (replacements kit including wiring for existing luminaires), etc. Accordingly, even though the following detailed example is related to a retrofit lamp, it is to be acknowledged by the skilled reader that this is merely intended as a non- limiting example.

With reference to Figs. 1 and 2, an illumination device 1 comprises a phosphor-based white light emitting diode 2 and a red light emitting diode 3 connected in series. The illumination device is connectable to a power source (not shown), such as a mains supply, for powering the light emitting diodes. The illumination device may further include one or more control units configured to control the input current or input power to the light emitting diodes. The phosphor-based white light emitting diode can e.g. be an InGaN LED coated with phosphor, such as YAG:Ce, SiA10N:Eu, (SrCaBaMgZn)SiNO:Eu,

(SrBaCa)Si:Eu, CaS:Ce.Cl, LiSrSiO:Eu, (SrCa)SiO:Eu, or combination of phosphors, and the red light emitting diode can e.g. be a GaP LED. However, other types of light emitting diodes are also feasible, such as AlGaAs, GaAsP, AlGalnP, organic or phosphor pumped. The white and red light emitting diodes are arranged so that their combined emitted light results in a pre-defined color point at normal operating temperature. This can be achieved by exactly combined light emitting diodes, or by adding an external adjustment resistor to achieve the desired color point.

The light emitting diodes 2,3 can be arranged on an electrically non- conductive substrate 5 provided with electrically conductive path(s) (not shown) within and/or fabricated on the substrate so as to connect the light emitting diodes 2,3 and any other electrical components, in any desired configuration. To enhance the heat dissipation, the illumination device preferably includes a heat sink 6, on which the substrate 5 is arranged, wherein the light emitting diodes 2,3 are thermally connected to the heat sink via thermal paths provided in the substrate. The thermal paths can be of copper or other suitable material of high thermal conductivity. For instance a conventional printed circuit board (PCB), such as e.g. an IMS-board (MCPCB), or a FR4-board, can be used as a substrate. Optionally, the illumination device may also include a light mixing chamber 11 for enhanced mixing of light emitted by the red and white light emitting diodes, a collimator (not shown) to shape the light beam and/or other optical element 12 such as a diffusor, or a lens as is well known in the art.

The illumination device further comprises at least one, and preferably two, positive temperature co-efficient (PTC) resistors 7, 8 electrically connected in parallel with the red light emitting diode 3. The PTC resistors are thermally connected to the red light emitting diode, and to the white light emitting diode such that the temperature of the PTC resistors changes with the junction temperature of the white and red light emitting diodes. Preferably, the arrangement is such that the junction of the white light emitting diode 2, the junction of the red light emitting diode 3, and the PTC resistors 7, 8 all have substantially the same temperature. However, as the red light emitting diode has the strongest temperature sensitivity, possibly even ten times higher than the white light emitting diode, the thermal contact between the positive temperature co-efficient resistors and the red light emitting diode is the most important. Also, when remote phosphors are used, the temperature of the remote phosphor can be substantially lower than that of the LED junctions. As schematically illustrated in Fig. 1, the PTC resistors 7,8 can be applied as first and second layers stacked on top of each other, with the white and red light emitting diodes 2,3 mounted on top of the upper layer relative to substrates, e.g. by means of soldering, gluing, mechanical clamping or any other feasible connection method. The thermal conductivity of the PTC resistors 7, 8 is preferably high so that the extra induced thermal resistance in the thermal path from light emitting diodes to the heat sink / ambient is low. As is recognized by a person skilled in the art, other arrangements are also feasible. For instance, the two PTC-layers 7, 8 can be positioned beside each other. The PTC resistors 7,8 can also be provided as conventional components that are arranged on the substrate 5 beside the white and red light emitting diodes 2,3 as long as the PTC resistors are thermally connected to the junctions of the white and red light emitting diodes. This can be achieved e.g. by a thermal path made of copper or other suitable material with a high thermal conductivity. Also, it is possible to configure the PTC resistor within the LED package, possibly substituting the TVS (transient voltage suppressor), implementing it as a layer or a block.

Fig. 3 is a diagram that illustrates typical temperature characteristics of the PTC resistors 7, 8. A first curve 9 illustrates how the resistance of the first PTC resistor 7 varies with the temperature, and a second curve 10 illustrates how the resistance of the second PTC resistor 8 varies with the temperature. The first and second PTC resistors have switching points 9' and 10', respectively, around which the positive temperature co-efficient resistors gradually switch from a low resistance state to a very high resistance state reducing the current flow to a very low level. Further, a third curve 16 corresponds to the combined resistance of the two PTC resistors. In the illustrated example, the resistance of the first PTC resistor 7 substantially coincides with the resistance of the second PTC resistor 8 at low temperatures, i.e. at temperatures below the switching points. This coinciding resistance value is not necessary, however.

In operation, when the illumination device 1 is switched on, the white light emitting diode 2 and the red light emitting diode 3 initially have junction temperatures about room temperature (i.e. about 25°C). At these junction temperatures, the efficiencies of the red and white light emitting diodes are relatively high, and thus the light output of the light emitted by the respective light emitting diode is relatively high. However, as the light emitting diodes emit light, heat is generated and the junction temperatures of the light emitting diodes rise until they reach their normal operating temperature, which is typically a junction temperature in the range 80°C to 110°C. The start-up period until the light emitting diodes reach their normal operating temperature may vary but is typically about five to fifteen minutes. Thus, during start-up, the rising junction temperatures reduce the efficiency of the light emitting diodes. In particular the efficiency of the red light emitting is reduced more than the efficiency of the white light emitting diode. However, as the light emitting diodes 2, 3 are thermally connected to the PTC resistors 7, 8; the temperatures of the PTC resistors 7, 8 are increased in response to the rising temperature of the junctions of the red and white light emitting diodes. As the temperatures of the PTC resistors reach their respective switching point 9', 10', the resistances of the PTC resistors increase so that more current is injected to the red light emitting diode. This higher input current to the red light emitting diode compensates for the larger efficiency reduction of the red light emitting diode compared to the white light emitting diode, thereby reducing the color shift during start-up. An additional, generally smaller but potentially significant effect may be that the color points of the light emitting diodes depend on current and temperature. Also this effect is

compensated by the parallel configured PTC resistor(s) and red LED(s).

A method of manufacturing the illumination device of Figs. 1-2 will now be described with further reference to Fig. 4.

In step 401, a phosphor-based white light emitting diode 2 and a red light emitting diode 3 are provided. The red and white light emitting diodes are here electrically connected in series and configured so that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature.

In step 402, at least one, and preferably two PTC resistors 7,8 are electrically connected in parallel with the red light emitting diode, and thermally connected to the white and red light emitting diode so that the temperatures of the PTC resistors change with the junction temperatures of the red and white light emitting diode, wherein the PTC resistors 7,8 are configured in such a way that the color shift relative to the predefined color point is less than a predetermined threshold during start-up, such as below 5 SDCM, more preferably below 3SDCM, or most preferred below 1.5 SDCM.

A method for selecting the PTC resistors 7,8 so that the color shift during start-up is less than a predetermined threshold will now be described with further reference to Fig. 5.

In step 501, a model of how light output of the phosphor-based white light emitting diode decreases with rising junction temperature is provided. Although more elaborate non-linear models can be used, the white light emitting diode is here approximated by a light output that decreases linearly with the junction temperature. For instance, the white light emitting diode can have a linear hot/cold factor of 0.94, defined for 25°C to 100°C, i.e. the light output of the white LED at 100°C is 94% of the light output at 25°C (for the same input current).Thus, the light output can be modeled by:

ø...,.... = ft. —.- ^■ i^'i - f 1 - 0,94) ^■ i T - 25V f lOO - 25Y) where 0^ _{its i} is the light output at 25°C.

In step 502, a model of how light output of the red light emitting diode decreases with the junction temperature is provided. Although more elaborate non- linear models can be used, the red light emitting diode is here approximated by a light output that decreases linearly with the junction temperature. For instance, the red light emitting diode can have a linear hot/cold factor of 0.604, defined for 25°C to 85°C, i.e. the light output of the red LED at 85°C is 60.4% of the light output at 25°C (for the same input current).

Thus, the light output can be modeled by:

O_rt£ = O_rii: · ( 1 - ( 1 - 0.604) · iT - : 5 := 55 - 25)) where 0,,_SiiS is the light output at 25°C.

Optionally, it is also possible to account for the change of the color of the light output of the red light emitting diode versus the junction temperature, e.g. by modeling the change of the dominant wavelength of the red light emitting diode, as:

Adom._^ j< ) = 612 - f„...— - -—— , where f^'^ is the current and T'^' is the temperature.

In step 503, a model of how the resistance of the PTC resistor varies with temperature is provided for each PTC resistor 7,8.

For instance for each PTC resistor the formula

can be used to model the resistance.

Here, dR/d _t and dR/άΤ are set to feasible constants, e.g. by studying the resistance vs. temperature behaviour of commercially available PTC resistors. For instance, =—0.O1 and = 0 , 2 for each of the PTC resistors 7,8.

Then, Ro, which is the resistance at room temperature, can be set to a value so that the illumination device provides a color point at room temperature that corresponds to the predefined color point at normal operating temperature. For instance, a resistance Ro of 275 Ω for the first PTC resistor 7, and a resistance of 220 Ω for the second PTC resistor 8.

Then, in step 504, the color shift during start-up of the illumination device is simulated using the models of the light emitting diodes and the PTC resistors by varying the temperature from room temperature to normal operating temperature, and determining the color point of the combined light emitted by the red and white light emitting diodes. This is done for different switching temperatures (T_sw) of the positive temperature co-efficient resistors 7,8, in order to determine, in step 505, the optimal switching temperatures for the PTC resistor(s), i.e. the ones that generates the minimal color shift. For instance, an optimization for 25°C to 90°C delivered as optimal parameters for the first PTC resistor a switching temperature of T_sw of 43°C, and for the second PTC resistor a switching temperature of 74°C. Optionally, tolerances of Ro and T_sw for the PTC resistors may be determined that allows the color shift during start-up to remain below a predetermined threshold, such as 5 SDCM, and more preferably 1.5 SDCM. Then, commercially available PTC resistors 7,8 can be selected to approximate the above determined ideal PTC resistors. The illumination device can then be assembled e.g. by a pick and place machine.

In order to enhance the quality of the illumination device and to discard fewer components, the method of manufacturing the illumination device may preferably include a method that finds combinations of components, from a batch of components, that maximize the fraction of the available components usable to manufacture illumination devices with a color shift below the predetermined threshold, as will now be described with further reference to Fig. 6.

In step 601 , a batch of white light emitting diodes, a batch of red light emitting diodes, a batch of first PTC resistors, and a batch of second PTC resistors are provided.

In step 602, properties of individual light emitting diodes and PTC resistors are measured. The measured properties for the light emitting diodes may include e.g. flux, color, voltage, hot/cold factor, whereas the measured properties for the PTC resistors may include e.g. the resistance at room temperature and the switching temperature.

In step 603, an optimization is performed by an optimizing computer program based on the measured properties to find combinations of the available light emitting diodes and PTC resistors that maximize the fraction of the available components usable to manufacture illumination devices with a color shift below the predetermined threshold.

Then, in step 604, illumination devices with a color shift below the predetermined threshold can be manufactured, e.g. by a pick and place machine, by picking and placing the components according to the resulting combinations found in step 603.

Each time a mishap occurs that renders a light emitting diode or positive temperature co-efficient resistor unusable, the picking and placing process can be stopped in step 605, and the optimization can be re-started in step 606 based on the, at that time, available light emitting diodes and PTC resistors. Then, in step 607, picking and placing of the components according to the resulting combinations of the new optimization can be continued.

It is noted that the measurement and the assembly process can be executed at different locations. For instance, collections of individually identifiable measured

components can be provided together with their measurement data (e.g. 'blue tape', a flat component carrier), so that the matching step can be executed immediately before final combining of the components into a subassembly, implying optimized matching at or near the pick and place machine, (mishap of a pick and place machine).

It is also possible to use a conventional binning process where the components are sorted into groups (or bins). For instance, the light emitting diodes can be sorted into different groups based parameters such as flux, color, voltage and hot/cold factor, and the PTC resistors can be sorted into groups based parameters such as resistance at room temperature and switching temperature. Then, during assembly of the illumination device, a phosphor-based white light emitting diode, a red light emitting diodes, a first PTC resistor and (preferably) a second PTC resistor are selected from appropriate groups and combined so that the color shift during start-up of the illumination device is below the predetermined threshold, e.g. 5 SDCM, or more preferably 1.5 SDCM. By binning the components based on their parameters, and combining components from appropriate bins, it is possible to provide an illumination device with a color shift below a predetermined threshold, such as 5 SDCM, using components that have such a wide range of parameter values that the resulting color shift of the illumination device from the not yet binned group would be larger than predetermined threshold.

Fig. 7a illustrates the color shift for an illumination device with two PTC resistors using the parameters given as exemplifying values in the description in relation to Fig. 5 above. As illustrated, the color drift during start-up is here reduced to less than 1.5 SDCM. This can be compared with Fig. 7b, which illustrates a simulation of the color shift of an illumination device without any positive temperature co-efficient resistor. Here, the color shift during start-up exceeds 20 SDCM. Further, Fig. 7c, illustrates a simulation where there is one positive temperature co-efficient resistor in parallel with the red light emitting diode. Here, the color shift during start-up can be reduced to about 4 SDCM.

Although the illumination device described above in relation to Figs. 1-7 uses a single white light emitting diode and a single red light emitting diode, the same principles are applicable in illumination devices with additional light emitting diodes. Fig. 8, schematically illustrates a circuit diagram for an illumination with a plurality of white light emitting diodes and a plurality of red light emitting diodes. Here, the white light emitting diodes 2 are connected in series to form a set of phosphor-based white light emitting diodes, and the red light emitting diodes 3 are connected in series to form a set of red light emitting diodes. Further, at least one, and preferably two positive temperature co-efficient resistors 7, 8 are electrically connected in parallel with the set of red light emitting diodes 3, whereas the set of phosphor-based white light emitting diodes 2 are electrically connected in series with set of red light emitting diodes 3 and the positive temperature co-efficient resistors 7, 8. Alternatively, when the PTC resistor is implemented as a layer, it may be advantageous to pattern PTC resistors for each light emitting diode.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, an alternative to connect the white and red LEDs in series is to separately control the current to the white LED and the current to the red LED. Further, the LEDs could be controlled to constant power instead of constant current. Further, it is possible to use more than two PTC resistors electrically connected in parallel with the red light emitting diode.

Claims

CLAIMS:

1. An illumination device (1), comprising:

a phosphor-based white light emitting diode (2) and a red light emitting diode (3) configured so that the combined light emitted by the phosphor-based white light emitting diode and the red light emitting diode has a pre-defined color point at normal operating temperature; and

a positive temperature co-efficient resistor (7) electrically connected in parallel with the red light emitting diode (3), and thermally connected to the white (2) and red (3) light emitting diodes such that the temperature of the positive temperature co-efficient resistor (7) changes with the junction temperatures of the white (2) and red (3) light emitting diodes,

wherein the positive temperature co-efficient resistor (7) is configured to reduce the color shift relative to the pre-defined color point during start-up of the illumination device below a predetermined threshold.

2. The illumination device according to claim 1, further comprising a second positive temperature co-efficient resistor (8) electrically connected in parallel with the red light emitting diode (3) and the first positive temperature co-efficient resistor (7).

3. The illumination device according to claim 2, wherein the resistance of the first positive temperature co-efficient resistor and the resistance of the second positive temperature co-efficient resistor substantially coincide in a temperature range below the switching temperatures.

4. The illumination device according to any one of the preceding claims, wherein the phosphor-based white light emitting diode (2) and the red light emitting diode (3) are connected in series.

5. The illumination device according to any one of the preceding claims, wherein the positive temperature co-efficient resistor (7,8) is a layer on which the white and/or red light emitting diode is arranged.

6. The illumination device according to any one of the preceding claims, further comprising at least one additional phosphor-based white light emitting diode (2') connected in series with the first phosphor-based white light emitting diode (2) to form a set of phosphor-based white light emitting diodes, and at least one additional red light emitting diode (3') connected in series with the first red light emitting diode (3) to form a set of red light emitting diodes, wherein the positive temperature co-efficient resistor(s) (7,8) is electrically connected in parallel with the set of red light emitting diodes, and wherein the set of phosphor-based white light emitting diodes are electrically connected in series with set of red light emitting diodes and the positive temperature co-efficient resistor(s) (7,8).

7. The illumination device according to any one of the preceding claims, wherein the phosphor-based white light emitting diode is an InGaN LED arranged to illuminate phosphor, and the red light emitting diode is a GaP, AlGaAs, GaAsP, AlGalnP, organic or phosphor pumped LED.

8. The illumination device according to any one of the preceding claims, further comprising an additional positive temperature co-efficient resistor arranged in parallel with both the phosphor-based white light emitting diode and the red light emitting diode and thermally connected to the light emitting diodes.

9. The illumination device according to any one of the preceding claims, wherein the illumination device is a retro-fit lamp.

10. A luminaire comprising the illumination device according to any one of the preceding claims.

11. A method of manufacturing an illumination device, comprising the steps of:

providing (401) at least one phosphor-based white light emitting diode (2) and at least one red light emitting diode (3) configured so that the combined light emitted by the at least one phosphor-based white light emitting diode and the at least one red light emitting diode has a pre-defined color point at normal operating temperature;

providing (402) at least one positive temperature co-efficient resistor (7) electrically connected in parallel with at least one red light emitting diode, and thermally connected to the white and red light emitting diodes so that the temperature of the at least one positive temperature co-efficient resistor changes with the temperatures of the red and white light emitting diodes, wherein the at least one positive temperature co-efficient resistor (7) is configured so that the color shift of the combined light emitted by the white light emitting diode and the red light emitting diode is below a predetermined threshold during start-up.

12. The method of claim 11, wherein the at least one positive temperature coefficient resistor is selected by the steps of:

providing (501) a model of how light output of the phosphor-based white light emitting diode varies with rising junction temperature;

providing (502) a model of how light output of the red light emitting diode varies with rising junction temperature;

providing (503) model(s) of how the resistance of the positive temperature coefficient resistor(s) varies with temperature;

using (504) the models to simulate the color shift of the illumination device during start-up by varying the temperature; and

optimizing (505) the parameters of the positive temperature co-efficient resistor(s) to achieve a color shift below the predetermined threshold.

13. The method of claim 12, wherein the parameters of the positive temperature co-efficient resistor(s) includes switching temperature (T_sw) and resistance (Ro) at room temperature.

14. The method of claim 11, further comprising the steps of:

providing (601) a set of available light emitting diodes and positive temperature co-efficient resistors;

measuring (602) properties of individual light emitting diodes and positive temperature co-efficient resistors in said set of available light emitting diodes and positive temperature co-efficient resistors;

performing an optimization (603) based on the measured properties to find combinations of said set of available light emitting diodes and positive temperature co- efficient resistors that maximize the fraction of the available light emitting diodes and positive temperature co-efficient resistors usable to manufacture illumination devices with a color shift below the predetermined threshold; and

assembling (604) the light emitting diodes and positive temperature coefficient resistors according to the resulting combinations to manufacture illumination devices with a color shift below the predetermined threshold.

15. The method of claim 15, further comprising the steps of:

stopping (605) the assembling process when a mishap occurs that renders a light emitting diode or positive temperature co-efficient resistor unusable;

re-starting (606) the optimization based on the, at that time, available light emitting diodes and positive temperature co-efficient resistors; and

continuing assembling (607) of the light emitting diodes and positive temperature co-efficient resistors according to the resulting combinations of the new optimization.